The present invention relates to secondary reflectors which are used in SHF antennae of the Cassegrain type. These antennae were first used in radar equipment, and are now widely employed in satellite communication systems, especially in individual terrestrial stations.
We are familiar with SHF antennae of the Cassegrain type, in which an SHF source placed on the axis of a main parabolic reflector illuminates a secondary reflector located close to the focus of this main reflector. The SHF wave is then reflected from this secondary reflector to illuminate the main reflector, and this allows a radiation diagram in the shape of a narrow beam to be obtained. This operation is reversed on reception of course.
The presence of the secondary reflector leads to a certain number of undesirable effects.
One of these effects is to mask a part of the surface of the main reflector, thus reducing the efficiency of the latter.
Another of these effects is a loss of part of the radiation reflected by the secondary reflector, which is diverted outside the surface of the main reflector. This xe2x80x9coverflow radiationxe2x80x9d, also known as xe2x80x9cspillover radiationxe2x80x9d, escapes as pure loss behind the antenna.
Great efforts have been exerted in order to reduce these undesirable effects by modifying the reflecting surface of the secondary reflector in relation to the initially hyperbolic shape of the optical Cassegrain telescope from which this type of SHF antenna was developed.
As shown in FIG. 1, a known SHF xe2x80x9csourcexe2x80x9d in such an antenna includes a circular wave guide (101) along which the SHF wave arrives. A hollow dielectric cone (102) is attached to this guide at one end and carries a secondary reflector (103) at the other end. The relatively complex shape of the surface of this reflector corresponds to the known state-of-the-art, so as to enable the aforementioned disadvantages, and the spillover radiation in particular, to be limited.
Even in this case, the dimensions of the secondary reflector, and therefore its masking effect, remain considerable. As a consequence, an increase in the dimensions of the main reflector is required in order to obtain the desired gain and directivity characteristics.
In addition, the overspill radiation that still remains, slight though it may be, reduces the performance of the antenna, and requires that it too must increase in size in correlation with the dimensions of the main reflector.
Now it is increasingly necessary, mainly for reasons of visual effect, to limit the size of antennae of this type, and this in turn requires an increase in the performance of the secondary reflector as well as a reduction in its size.
In order to achieve these effects, the invention proposes a secondary reflector for SHF antennae of the Cassegrain type which includes a basic secondary reflector consisting of a first circular xe2x80x9cringxe2x80x9d in the shape of a cylinder made of conducting material, whose diameter is equal to the external diameter of the basic reflector, secured by one of its ends to the outer edge of this basic reflector so as to project from the side of the reflecting surface of the reflector, and whose height (H) is designed to reduce the xe2x80x9coverspill radiationxe2x80x9d of the secondary reflector.
The invention is characterised in that the reflector also includes a second xe2x80x9cringxe2x80x9d in the shape of a circular crown, also made of conducting material, whose inside diameter is equal to the diameter of the first ring, fixed to the free end of this first ring, and with a width (h) that is designed to further reduce the aforementioned overspill radiation.
According to another characteristic of the invention, the values of parameters H and h are of the order of one quarter of the average wavelength for which the antenna is dimensioned.
According to another characteristic of the invention, the first and the second rings are made in the shape of a single full ring of height Hxe2x80x2 and thickness hxe2x80x2, and the reflector consists of a cone made of a solid dielectric material, which connects the waveguide designed to feed into the antenna at the basic reflector, in order to allow the values of parameters Hxe2x80x2 and hxe2x80x2 to be reduced in relation to the values of parameters H and h.
According to another characteristic of the invention, the free end of the single full ring is machined so as to present a cut-away which reduces the thickness of its outer circumference in order to further reduce said overspill radiation.